Polyethylene (xe2x80x9cPExe2x80x9d) has many desirable mechanical properties and it is readily synthesized, and manufactured in any desired shape and size. In particular, there are many uses for PE, in its several grades, and particularly for high density polyethylene (xe2x80x9cHDPExe2x80x9d) in the form of tubing, pipes, conduits, and the like. For ease of reference, the use of the term, xe2x80x9cpipexe2x80x9d or xe2x80x9cpipingxe2x80x9d in the singular or plural herein, should be understood to also encompass any other configuration of tubing or conduit, and the joiner and/or connector components, such as straight joints, elbow joints, end-caps and the like, unless otherwise specified.
It is also known to the art that many potential uses for pipe comprising PE, in whole or in part, have previously been impractical due to the inherent limitations of this polymer material. This is of particular concern in the manufacture of extruded, pre-insulated pipes for general industry, the building trades, ocean platforms, e.g., offshore oil and gas platforms, and ship building. In all of those environments, there is a demand for insulated pipes. Generally, pipes that are pre-insulated during the manufacturing process are more economical to produce and install. One preferred type of pre-insulated pipe has an inner carrier pipe, manufactured from any art-known material such as, for example, mild steel, stainless steel, PE formulated with any art-known copolymer (xe2x80x9cPEXxe2x80x9d) and/or HDPE, and the like. This inner carrier pipe is jacketed with a foam insulating layer, e.g., a hard polyurethane foam, that is, in turn, jacketed by a protective outer shell, preferably of HDPE, although steel is used for some applications. Optionally, one or more additional layers of foam, or other insulating materials, e.g., mineral wool or fiberglass or similar material, can also be incorporated into the structure of such insulated pipe. This type of pipe, with an HDPE outer shell, is typically manufactured as a single unit by an extrusion process.
HDPE pipes, including insulated pipes with an HDPE outer shell, are economical to manufacture and install, light, strong, and corrosion resistant. Of particular importance for the ship-building industry, pre-insulated extruded HDPE pipe is more resistant to penetration of moisture into the insulating layer than are conventional insulated pipes. However, there are obstacles to wider use of this type of pre-insulated pipe manufactured solely from polymer materials. The most important obstacle is that pipe manufactured from conventional PE-based polymers, including HDPE, is generally unsuitable for use in areas where flame retardancy is required. For example, the melting point for HDPE is about 120xc2x0 C. When exposed to sufficient heat for even a brief period of time, HDPE readily melts and forms burning drops which can spread fire and/or cause severe burns on contact with human skin and clothing. Once ignited, HDPE bums intensely, producing noxious gas and smoke.
Other shortcomings of conventional HDPE pipes, whether insulated or single wall, relate to difficulties in adhering coatings to HDPE. PE-based polymers such as HDPE are among the most difficult polyolefins to successfully coat with a paint or glue, and there is a long-standing need in the art for a solution to this problem.
Paintability and/or gluing onto a surface is considered to require a minimum surface energy of at least 38 dynes/cm2. In fact, it is considered that successful adhesion with most standard paints generally requires a surface energy over 50 dynes/cm2, but HDPE is reported to have a surface energy of only about 31 dynes/cm2. A surface energy of 80 dynes/cm2, or higher, is considered to be best for optimum post-manufacture painting, marking or adhesion, particularly when the paint or adhesive must remain on the surface for many years, while withstanding extreme environmental conditions.
The requirement for painting relates to the practical and economic importance of color coding for identification of specific components in the construction and maintenance of tubing, pipes and/or conduits in a complex installation. The requirement for gluing and adhesion relates, for example, to the practical need to attach other construction materials, as well as markings and/or indicia of various kinds, to the pipe after installation.
Previous efforts to address some of these shortcomings in HDPE pre-insulated pipes have required the use of a metal-jacketed pre-insulated pipe for the outer shell to provide flame retardancy and paintability. However, the use of a metal jacket, e.g., steel, as the outer shell adds weight and cost to manufacture and installation, among other limitations.
Another way in which the surface properties of materials formed of PE-based polymers have previously been enhanced is by blending other polymers with the stock polyethylene, before extrusion, to impart flame retardant properties and/or paintability. For example, various products are commercially available in the form of granules, which, when blended with HDPE during manufacture, impart some protection against heat and flame. However, they all have the disadvantage that they change the mechanical properties for polyethylene. In addition, the processing requirements of blending other polymers into the HDPE adds to the costs of materials, and requires custom manufacture, which makes it difficult to economically supply pipe as required by the end user in the various industries.
Yet another problem with conventional PE compositions is its sensitivity to degradation by ultraviolet radiation in outdoor applications. The normal color of polyethylene is a milk-white. One way that PE has previously been protected from ultraviolet radiation in outdoor applications is by the addition of carbon black to the composition. Of course, this does not help any of the other shortcomings of PE, and for outdoor applications, a black PE composition can experience excessive heating from the infrared portion of sunlight, particularly with outdoor use in regions with a high solar flux, such as the tropics.
Another possible method for enhancing surface properties of polymeric materials is by grafting or bonding coatings onto the surfaces in need of such treatment. Polymers have been grafted onto non-metallic substrate surfaces, such as cellophane, and polyolefins. For example, U.S. Pat. No. 5,753,557 describes grafting of a polymer onto cellophane using ultrasound to cure the grafted layer. U.S. Pat. Nos. 5,342,659 and 5,232,748, describe grafting of a polymer onto cellophane using radiation energy, e.g., microwave, laser or ultrasonic energy to cure the grafted layer.
U.S. Pat. No. 3,880,580 describes methods of activating polymer substrates such as polyolefins, polypropylene, polyester and polyvinylchloride materials to accept sulfonation in the presence of a chlorinating hydrocarbon solvent, so that the grafted substrate can be dyed.
U.S. Pat. No. 5,407,728 describes a solution for grafting a polymer onto fabrics, such as polyamides, polyaramids or polyesters, to enhance flame retardant properties and resistance to fraying, for use in the manufacture of fabrics, including fabrics for airbags. As described by U.S. Pat. No. 5,741,548, polymers can be grafted onto fabrics, including polyethylene fabrics, to enhance the printability of satin and reemay acetate fabrics.
The descriptions of the patents mentioned above provide grafting solutions for very specific purposes, which cannot be readily generalized to other materials. Nowhere do any of these patents teach or suggest grafting onto solid PE or PE-based polymers, to provide for enhanced flame retardant properties and other improved surface properties in pipes, beams and other similar articles of manufacture. For instance, U.S. Pat. No. 5,407,728 teaches the need to avoid excessive crosslinking in the graft coating to prevent undesirable stiffness, whereas in the graft coating of a solid substrate a high level of crosslinking is typically desired in order to enhance such properties as, e.g., flame retardancy and scratch resistance.
Thus, there remains a longstanding need in the art for methods and compositions for grafting selected coatings onto PE, including HDPE and other PE-based polymers, to provide improved surface properties, including flame retardancy, ease of painting, scratch and abrasion resistance with a surface energy of more than 80 dynes/cm2, and other improvements.
Accordingly, the present invention provides methods and compositions for graft modifying the surfaces of PE, including HDPE and other PE-based polymer compositions. The inventive grafting process employs polyfunctional monomers/prepolymers, such as, for example, vinyl monomers, urethane and epoxy prepolymers which are chemically bonded to the PE surface by the grafting process provided herein.
Broadly, the invention provides for a graft coated substrate that includes one or more types of PE, wherein the graft coating is covalently bonded to the substrate, and the coating includes a non-polyethylene polymer or copolymer, such as a vinyl polymer, a urethane, an epoxy, a polysilicone and/or combinations thereof. Optionally, the graft coating also includes a pigment or colorant, a fire retarding agent, and/or combinations thereof
In one embodiment, the substrate includes a PE having a density, for example, ranging from about 0.930 g cmxe2x88x923 to about 0.940 g cmxe2x88x923, or greater. The included polyethylene broadly has an average molecular weight ranging, e.g. , from about 100,000 amu to at least 6xc3x97106 amu.
In another embodiment, the graft coated substrate is optionally a solid PE. By xe2x80x9csolidxe2x80x9d is meant a PE substrate that is not in the form of a filament, yarn, fabric or cloth material.
Thus, the substrate is preferably formed into an article of manufacture, either before or after the graft coating is applied to the substrate. The article of manufacture is any article suitable to be manufactured from material that includes a PE. Simply by way of example, the article of manufacture is advantageously a pipe or tube, a curved or planar sheet, a beam, a board, a rod or shaft, a container for solids or fluids, and/or combinations thereof
Graft coated pipe according to the invention includes, for example, straight pipe, bent pipe, a straight pipe joint, an elbow joint, an end-cap, a heat-shrinkable joint, and combinations thereof. The graft coated pipe according to the invention also includes, for example, single wall pipe, pipe with a plurality of walls nested one within the other, pipe with a single insulating layer between two concentric walls, and pipe with a plurality of concentric insulating layers, to name but a few types of pipe that will benefit from the graft coating compositions and methods of the invention.
The graft coating provides polyethylene substrate with a number of improved properties, including the ability to resist melting and burning for a time period ranging from about 1 to about 18 minutes. This test is configured so that the article is exposed to a planar heated surface that is heated, e.g., by burning fuel (e.g., natural gas or propane), or electrical resistance, to a temperature ranging from about 800 to about 960 degrees C. The heating panel is a rectangle that measures about 25xc3x9751 cm, and the graft coated substrate that is tested is positioned at a distance of about 12.5 cm from the heating panel.
Further, the graft coating provides the polyethylene substrate with an improved surface energy allowing for post manufacture painting, coating, marking and adhesive application to the grafted surface. The graft coated substrate according to the invention has a surface energy ranging from about 56 to about 80 dynes/cm2, or higher.
Processes for applying the graft coating to polyethylene substrates are also provided by the invention. One process according to the invention is a method for modifying the surface of a solid polyethylene substrate by covalently grafting a heat resistant coating onto the substrate by
(a) applying to a PE substrate, e.g., a solid PE, a liquid composition that includes effective amounts of a monomer or prepolymer, a graft initiator, a catalyst and a polymerization promoter, under conditions effective to promote grafting of the monomer or prepolymer to the solid polyethylene substrate, to form a coating on the substate, and
(b) curing the applied composition.
Optionally, the liquid composition of step (a) includes a pre-formed polymer, suitable to be grafted to the activated substrate surface, alone and/or in combination with one or more of the monomer/prepolymers. The polymer is, e.g., a vinyl polymer, a urethane, an epoxy, a polysilicone, and/or combinations thereof, suitable to be grafted to the PE surface. In a further optional embodiment, the liquid composition also includes a colorant such as a dye or pigment, and/or a fire retardant agent.
In another embodiment of the invention, the liquid composition is first prepared without the polymerization promoter, and the process further comprises the step of mixing the polymerization promoter with the liquid composition prior to application of the liquid composition to the substrate, which allows for a longer storage period for the prepared liquid composition.
The monomer or prepolymer is a vinyl monomer, a urethane monomer, an epoxy monomer and/or a silicon-based monomer or prepolymer. The graft initiator is an effective amount of a metal ion, e.g., present in a concentration ranging from about 0.01 to about 1.0%, by weight. For example the metal ion is an ion of iron, silver, cobalt, copper, cerium and/or combinations thereof. The catalyst is a peroxide present in the liquid composition in a concentration ranging from about 0.1 to about 5% by weight and includes, e.g., benzoyl peroxide, methyl ethyl ketone peroxide, 1-butyl hydroperoxide and/or combinations thereof. The process of claim 13 wherein the polymerization promoter is present in a concentration effective to react with, and crosslink, the monomer or prepolymer. The polymerization promoter is a polyfunctional aziridine liquid crosslinker.
In yet a further embodiment, the grafted substrate is a polyethylene having a density ranging, for example, from about 0.930 g cmxe2x88x923 to about 0.940 g cmxe2x88x923, or greater.
Optionally, the applied composition is self-curing, and/or cured by heating, and/or by exposure to ambient atmospheric moisture, e.g., when the monomer or prepolymer is a moisture curing (e.g., a moisture curing urethane). Depending upon the required conditions, the applied graft coating is cured at room temperature, e.g., for a period of time as long as 6 days, or by the application of heat, e.g., ranging up to about 200 degrees F. for a time period of as little as 30 minutes.
In a further still embodiment, the liquid composition further comprises a compatible flame retardant agent, such as a phosphorous-based flame retardant.
In any of the previously mentioned embodiments, the liquid composition is applied to the substrate by a method selected from the group consisting of brushing, dipping, spraying and/or combinations thereof.
In yet a still further composition, the invention provides for a solid polyethylene substrate, e.g., an article of manufacture such as an extruded polyethylene pipe, that includes a graft coating covalently bonded thereto, prepared by the process the above described methods.